During the conversion of peat to coal, methane gas is produced as a result of thermal and biogenic processes. Because of the mutual attraction between the coal surface and the methane molecules, a large amount of methane can remain trapped in-situ. The reserves of such "coalbed methane" in the United States and around the world are huge.
Conventional coalbed methane recovery methods are based on reservoir pressure depletion strategy; that is, methane is desorbed from the coal surface by reducing the reservoir pressure in the coal cleat network. Thus, both water and methane gas are recovered simultaneously from a coalbed. While this method of coalbed methane production is simple, it is not efficient. Loss of reservoir pressure deprives the pressure depletion process of the driving force necessary to flow methane gas to the wellbores. Consequently, the gas production rate from a well is adversely affected by the reduction in reservoir pressure.
Another method of recovering coalbed methane is by injecting into the coal seam a gas, such as C0.sub.2, having a higher affinity for coal than the adsorbed methane, thereby establishing a competitive adsorption/desorption process. In this process, the C0.sub.2 displaces methane from the surface of coal, thereby freeing the methane so that it can flow to a wellbore and be recovered. This method is disclosed in the reference by A. A. Reznik, P. K. Singh, and W. L. Foley, "An Analysis of the Effect of C0.sub.2 Injection on the Recovery of In-Situ Methane from Bituminous Coal: An Experimental Simulation," Society of Petroleum Engineers Journal, October 1984. The problem with this method is the large volume of C0.sub.2 that must be injected into the coal seam in order to exchange sites with methane. In most coal seams, such an amount would be uneconomical. This reference reports that mixing even small amounts of nitrogen gas with C0.sub.2 significantly reduces the effectiveness of displacement desorption of methane by C0.sub.2.
There is a need for a method of producing coalbed methane from coal that accelerates the production rate and improves recoverable gas reserves economically.